148 EVOLUTION 



of the organism counts for more than the 

 conditions. 



Changed habits, e.g. changes in the de- 

 gree of use or disuse of a part, produce an 

 inherited effect, witness the lighter wing- 

 bones and heavier leg-bones of the domestic 

 duck, or the enlarged udders of cows. 



Variations are often definitely correlated: 

 thus short-beaked pigeons have small feet; 

 hairless dogs have imperfect teeth; and 

 blue-eyed white tom-cats are deaf. Hence 

 selection of any one character will prob- 

 ably modify others indirectly. 



Although the laws of inheritance are 

 mostly unknown, it seems that probably 

 most, if not all, characters tend to be in- 

 herited. There is no satisfactory evidence 

 to support the popular idea that domestic 

 varieties revert to the primitive stock when 

 they run wild. Reversions occasionally 

 occur in domestication, but there is no gen- 

 eral tendency to lose what has been gained 

 apart, of course, from breeding with wild 

 stocks, or with other domesticated ones. 



Except in being less uniform than natural 

 species, in often differing more widely in a 

 single part, and in being fertile when crossed, 

 there are no well-marked distinctions be- 

 tween our domestic races and the so-called 

 true species of a genus. The many breeds of 



